‘This is too easy,” Barack Obama is recorded as saying in “Shattered,” an exhaustive account of Hillary Clinton’s ill-fated 2016 presidential campaign. The president had just delivered a well-received speech in praise of Mrs. Clinton’s candidacy; both Mr. Obama and Mrs. Clinton had derided Donald Trump, the Republican nominee, as a malignant nincompoop. John Podesta, the Clinton campaign’s chairman, looked at the president doubtfully. Too easy? “All right, all right, all right,” Mr. Obama playfully conceded. “There’s just so...